I have 3 Saitos, including a 72. It runs like a charm all the time and it's inverted. Same goes for the 91. I have a 56 but it's upright. I love all of them. They start very easily, are very reliable, and they pull like an ox. Plus, they sound cool. Go with the four stroke.

I agree with the 14x4 prop for 3D. A 13x6 will work well too.